GRAVIMETRIC METHODS

Gravimetric methods of which are based upon the measurment of mass, are of two major types: Precipitation Methods Volatilization Methods

Properties of Precipitates A.Easily filtered and washed free of contaminants B.Sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing C. Unreactive with constituents of the atmosphere D. A known chemical composition after it is dried or, if necessary, ignited

Factors that determine the particle size of precipitates: Relative supresaturation Q = concentration of a species at any instant S = equilibrium solubility

Mechanism of precipitate formation: The precipitates form by two different pathways: Nucleation ( increase enormously with increasing relative supersaturation ) Particle growth ( increase moderately with increasing relative supersaturation )

Experimental control of particle size: Dilute solutions (to minimize Q) Slow addition of the precipitating agent with good stirring Control of acidity

Colloidal precipitates: A) Coagulation of colloids occurs by: Heating Stirring Adding electrolyte B) Peptization of colloids

The electric double layer:

Surface adsorption of ions on colloid:

Effect of adding electrolyte:

Crystalline precipitates: Methods of improving particle size and filterability: Minimization of Q by using dilute solutions and adding the precipitating reagent slowly and good mixing Maximizing of S by precipitation from hot solutions or by adjusting the pH of precipitation medium

Types of Coprecipitation a) Surface adsorption Mixed – crystal formation Occlusion and mechanical entrapment b) Coprecipitation errors : negetive and positive errors

Surface area of colloids:

A coagulated colloid:

Precipitation from homogeneous solution:

Drying and ignition of precipitates

Application of gravimetric methods: Inorganic precipitating agents Reducing agents Organic precipitating agents

Some organic precipitating agents: